Traditionally, continuous casting of light metal ingot has followed the practice of introducing molten metal into one end of an open-ended mold and withdrawing a solid or partially solidified ingot from the opposite end. Typically, the casting mold is relatively short in the axial direction and is hollow or otherwise adapted to receive a liquid cooling medium, such as water, which chills and solidifies the ingot meniscus. The water is then discharged from the mold and continues to chill the ingot as it contacts the ingot surface. Molds are preferably constructed of aluminum but may also be copper, bronze or another material which exhibits high thermal conductivity.
U.S. Pat. No. 4,166,495 issued to Yu discloses an ingot casting method for controlling the withdrawal of heat from the surface of a cooling ingot. The essence of the invention of U.S. Pat. No. 4,166,495 is the mixing of a gas with the liquid coolant before the liquid coolant is applied to the ingot surface. When the gas containing liquid coolant is applied to the mold during the initial stages of casting, the gas mixed in the liquid coolant acts to retard the rate of heat extraction of the liquid coolant. When the amount of gas mixed with the liquid coolant is reduced, the rate of heat extraction by the mold is increased. The increased rate of heat extraction is used on subsequent portions of the emerging ingot length.
The method of U.S. Pat. No. 4,166,495 is a commercially successful method of retarding the cooling effect of the liquid coolant and has come to be known in the aluminum industry as the Alcoa 729 process. The Alcoa 729 process has been successfully used to cast ingot having a width to thickness ratio of 5.3.
This is the largest width to thickness ratio that the applicant knows is commercially feasible for casting ingot and is obtainable only by using the Alcoa 729 process. A preferred coolant of the Alcoa 729 process is water and one preferred gas is CO.sub.2. Water is a preferred coolant because it is inexpensive and abundant. CO.sub.2 is preferred because it is odorless, inexpensive, highly soluble in water and relatively harmless to the environment. In addition, since there is no gaseous buildup in the recycled water, CO.sub.2 does not suffer from many of the disadvantages associated with chemical additives. Other gases which are substantially insoluble in water, such as for example air, may also be used in practicing the method of U.S. Pat. No. 4,166,495.
U.S. Pat. No. 4,693,298 issued to Wagstaff discloses a means and technique for casting metals at a controlled direct cooling rate. The method of U.S. Pat. No. 4,693,298 involves mixing liquid coolant and a gas which is substantially insoluble in the liquid coolant by discharging the gas through jets. The jets release the gas in the flowing liquid coolant as a mass of bubbles that tend to remain discrete and undissolved in the coolant as the coolant on the surface of the ingot.
Although the Alcoa 729 process is economical and effective, it is improvable. The amount of gas that must be mixed with the liquid coolant in the process is very sensitive to changes in temperature, mixing pressure, and water quality. The ability of the gas to retard the heat of extraction of the liquid coolant is determined by the volatility of the liquid, which depends on the concentration of gas mixed in the liquid coolant, the temperature of the liquid coolant, the velocity of coolant flow and the coolant quality of the liquid coolant. The term "quality" as used herein means the chemistry of the liquid coolant and it includes properties such as ph, alkalinity, dissolved and suspended solids, surface tension and ionic species. The sensitivity of the process to changes in temperature and coolant quality requires that the amount of gas that is added to the cooling medium must be constantly adjusted by trial and error so that the desired heat transfer characteristics of the liquid coolant can be maintained. The temperature sensitivity and coolant quality sensitivity of the Alcoa 729 process makes casting of some ingots extremely difficult.
Accordingly, it would be advantageous to provide an economical and effective method of monitoring and controlling the cooling effect of the liquid medium at a desirable level even though the coolant quality and temperature may vary.
The primary object of the present invention is to provide a method and apparatus for monitoring the cooling effect of the liquid medium containing gas bubbles.
Another object of the present invention is to provide a method and apparatus that can be readily added to existing casting facilities which permits a greater control of the rate of heat extraction from a liquid coolant in which gas has been mixed to regulate the cooling rate of the coolant when it is used to cool the surface of a continuously cast ingot.
A further object of the present invention is to provide a method and apparatus for continuously casting ingots that can compensate for changes in the heat transfer characteristics of the cooling medium due to changes in the temperature and coolant changes without dramatic variations in the flow rates of the liquid cooling medium.
Another object of the present invention is to provide a method and apparatus that can be readily added to existing casting facilities which permits a monitoring of the size and concentration of the gaseous bubbles in a liquid medium that is used to cool the surface of a continuously cast ingot.
Yet another object of the present invention is to provide a method and apparatus that can be readily added to existing casting facilities which permits the casting of ingot with a larger width to thickness ratios than has heretofore been commercially feasible.
Another object of the present invention is to provide a method and apparatus that can be readily added to existing commercial casting facilities which permits the casting of ingot with an aspect ratio greater than 5.3.
Still another object of the present invention is to provide a method and apparatus that can be readily added to existing casting facilities which reduces the severity of the most common deformities occurring on the butt end or the initially emerging bottom surface of a continuously cast ingot.
Still another object of the present invention is to provide a method and apparatus that can be readily added to existing casting facilities which will minimize ingot butt curl on the butt end or the initially emerging bottom surface of a continuously cast ingot.
Still a further object of the present invention is to provide a method and apparatus that can be readily added to existing casting facilities which will increase the safety of ingot startup by minimizing the possibility of an explosive bleed out in the butt end or the initially emerging bottom surface of a continuously cast ingot.
Still another object of the present invention is to provide a method and apparatus that can be readily added to existing casting facilities which will (a) monitor and control the transition from a liquid coolant containing gaseous CO.sub.2 to a non-gaseous liquid coolant and (b) minimize the ingot deformation that is normally associated with the transition.
These and other objects and advantages of the present invention will be more fully understood and appreciated with reference to the following description.